Assume that an actual yaw rate of a vehicle has a relationship of first order delay to a standard yaw rate of the vehicle and a coefficient multiplied to a vehicle speed in a time constant of the first order delay is referred to a time constant coefficient of steering response. A stability factor of the vehicle and a time constant coefficient of steering response t represent a turning characteristic of the vehicle. A stability factor of the vehicle and a a steering-response-time constant coefficient can be estimated by using ARX (auto-regressive exogenous model) to estimate parameters a and b of a discrete-time transfer function from a standard yaw rate of the vehicle to an actual yaw rate of a vehicle.
In, for example, Japanese Patent Application Laid-Open (kokai) No. 2004-26073, a turning characteristic estimating device for a vehicle is described which estimates a standard yaw rate of a vehicle on the basis of running data when the vehicle turns; estimates parameters a and b of a discrete-time transfer function from a standard yaw rate of the vehicle to an actual yaw rate of a vehicle; estimates a time constant coefficient of steering response on the basis of the parameter a; and estimates a stability factor of the vehicle on the basis of the parameters a and b.
In conventional turning characteristic estimating devices for vehicle such as that described in the above-mentioned laid-open publication, a lot of data of standard yaw rates and actual yaw rates are required to estimate the parameters a and b. For this reason, there is a problem that it takes a long time to estimate a stability factor of the vehicle and a time constant coefficient of steering response.